Morphometry and Structure of Eroded Complex Impact Craters: a Parameter Study Us- Ing Hydrocode Modeling

Introduction: Most impact structures on Earth are modified by erosion. First order structural features of impact craters such as the crater diameter, the size of the central uplift, and ring syncline can be measured in field campaigns [e.g. 1] or by geophysical explorations [e.g. 2,3]. However, these quantities deviate from the original crater diameter or other morphological features of the pristine crater. Here we present first results of a systematic numerical modeling parameter study of crater formation addressing the question how these structural features develop as a function of depth for a range of crater sizes. Our objective is to determine size-ratios between features such as the diameter of the ring syncline and the rim-to-rim diameter and how they vary with depth at complex impact structures. By comparing the model-derived ratios with observed structural data of natural craters it may be possible to better constrain the amount of erosion of terrestrial impact craters. Model Setup: We used the iSALE 2D hydrocode [4,5] to simulate complex impact craters in the size range between 10 and 50 km. Projectile sizes were varied between 600-3000 m. Other model parameters were kept constant for all modeled crater sizes. We made the models as simple as possible (vertical impact, g = 9,81 m/s, impact velocity U = 15km/s) and assumed both projectile and target composed of granite. The thermodynamic state was calculated with Tillotson’s equation of state [6]. The strength behavior of the material was described by a Drucker-Prager model, where strength Y is linear function of pressure P (Y=C+μP, where μ is the internal friction coefficient, and C the cohesion). To simulate a temporary strength degradation of the target during crater formation we applied the acoustic fluidisation model [7,8,9]. Methods: In a first step we calibrated our models to find the “right” set of acoustic-fluidisation parameters [9]. We chose two relatively well preserved terrestrial impact structures on Earth with different size and tried to reproduce crater morphometry by keeping the acoustic fluidisation parameters constant. The best fit was found for a decay time of β = 120 and a viscosity of γ = 0,004. Note that these parameters represent a compromise to enable modeling of impact craters over a considerable range of size. For the calibration we chose the Bosumtwi (Ghana) [1] (crater diameter ~ 12 km, width of central uplift ~ 1.8 km, max. high of central uplift ~ 200 m) and Chesapeake Bay (USA) [2] (crater diameter ~ 40 km, width of central uplift ~ 10 km, max. high of central uplift ~ 2000 m) impact craters. Note that we modeled only the inner crater of Chesapeake Bay and neglected the extensive enlargement of the crater due to an extremely weak, water–saturated sediment layer as the uppermost unit [10]. In a second step we modeled craters in a size range between 10 and 50 km by varying the projectile diameter between 600 m and 3000 m in steps of 200 m so that we finally got 13 numerical models. For each model we determined the apparent crater rim diameter and ring syncline diameter as a function of depth. The trace of the crater rim with increasing depth (Fig. 1) is defined by a sudden change in dipping of marker lines and an increase in plastic strain. The ring syncline is defined here as a local depression of a marker line between the central uplift and the crater rim. The ring syncline axis is located at the deepest point of the depression. The trace of the ring syncline with increasing depth connects the deepest points of each marker line (Fig. 1). First results: Fig. 2-4 show the course of the apparent crater diameter and ring synclines as a function of depth for different crater sizes. As expected, all models show a decrease of the apparent crater diameter (Dr) with depth (at different levels of erosion) in a similar manner (Fig. 2). The S2